The invention relates to electrical engineering, in particular, to capacitor-making industry and can find application in producing high-capacity electric capacitors making use of double electric layer (DEL). Capacitors with DEL have found application as stand-by power sources in systems requiring uninterrupted power supply, such as computation engineering, communications equipment, numerically controlled machine tools, in uninterrupted cycle production processes; for electric-starter starting of diesel engines; for power supply of invalid carriages, golf carriages, etc.
Known in the art presently are electric energy accumulators appearing double electric layer (DEL) capacitors, e.g., those disclosed in U.S. Pat. No. 4,313,084 (1982) and 4,562,511 (1985). Said capacitors comprise each two porous polarizable electrodes and porous separator made of a dielectric material and placed therebetween, and current leads. A liquid electrolyte solution in the capacity of which either aqueous or non aqueous electrolytes are used (is contained in the pores of the electrodes and separator, as well in a certain free space inside the capacitor casing. Electric charge is accumulated on an interphase surface in the pores between the electrode material and the electrolyte. Used as the materials of polarizable electrodes are various routine porous carbon materials. To increase the capacitor capacitance with double electric layer said carbon materials are subjected to preactivation with a view to increasing their specific surface area up to 300-3000 sq.m/g.
DEL capacitors have much higher capacity compared with routine film-type and electrolytic capacitors, i.e., up to a few scores of Farads per gram of active electrode materials. However, such capacitors suffer from a rather specific energy, i.e., not more than 3 watt-hours/lit.
Another disadvantage inherent in DEL capacitors resides in evolving gases during recharging, e.g., oxygen on the positive electrode and/or hydrogen on the negative electrode. The fact is due to the fact that the potential of evolution of said gases on the respective electrodes is reached during recharging. The result is an increased pressure of the gases inside the capacitor casing which may lead to its depressurization and even blasting if no provision is made for a special pressure release valve. However, operating reliability of such valves is frequently inadequate to ensure against such depressurization or blasting due to their getting clogged with dirt, and so on. It is for said and other reasons that DEL capacitors suffer from a fundamental disadvantage, that is, a danger of their depressurization and even blasting which involves special service and maintenance thereof. To provide more reliable prevention of depressurization during recharging, one should considerably reduce the final charging voltage for the sake of xe2x80x9cdouble insurancexe2x80x9d, whence the initial discharge voltage is reduced, too so as not to approach a dangerous border-line. This in turn results in a considerable drop of the specific energy of the DEL capacitor which, as is commonly known, is directly proportional to the squared specific energy of the DEL capacitor which, as is commonly known, is directly proportional to the squared difference between the initial and final discharge voltage values.
Known in the present state of the art is a DEL capacitor (cf. WO 97/07518 of Feb. 27, 1997) having a polarizable electrode made of a porous carbon material, and a non-polarizable electrode made of nickel oxide. Used as electrolyte is an aqueous carbonate or hydroxide of an alkali metal. Such a capacitor gives much more specific energy value compared with a DEL capacitor having two polazable electrodes (up to 45 J/cu.cm or 12.5 W-h/lit), and a maximum voltage of 1.4 V. However, the capacitor described before yet suffers from substantial disadvantages, that is, the problem of how to provide its complete pressurization and need in special service and maintenance thereof. As a result of non-provision of a complete pressurization of the capacitor are reduced values of a maximum charging voltage and specific energy, as well as inadequately high charging current values and hence too long a charging time.
It is an object of the present invention to provide a completely pressurized servicexe2x80x94and attendance-free capacitor.
It is another object of the invention to enhance the specific energy of the capacitor and to reduce the charging time.
The foregoing objects are accomplished due to the herein-disclosed invention whose essence resides in that in a dual electric layer capacitor comprising two electrodes of which either one or both are polarizable, a liquid electrolyte, and a separator, the degree of filling the void space of the separator and of both electrodes with electrolyte falls within 90 and 40%.
The essence of the present technical solution resides in that gaseous oxygen liberated on the positive electrode of a DEL capacitor at the end of charging and during recharging is basically absorbable completely on the negative electrode during its ionization reaction (electric reduction) due to very high polarization of said reaction (Ep greater than 1 V) and due to the fact that the activated carbon is a very good catalyst for the process in question, whereby it is made use of in fuel cells (cf. xe2x80x9cChemical current sourcesxe2x80x9d by V. S. Bagotski and A. M. Skunden, Moscow, xe2x80x9cEnerghiaxe2x80x9d PH, 1981, pp. 80, 116 (in Russian). On the other hand, gaseous hydrogen which can be liberated on the negative electrode during recharging a DEL capacitor, can substantially be absorbed completely on the positive electrode during its ionization reaction (electric oxidation) due to a very high polarization of said reaction (Ep greater than 1 V). However, in routine DEL capacitors the pores of the separator and of both electrodes are filled with electrolyte virtually completely so that gas porosity in said porous bodies is virtually absent. Under such conditions very much trouble is encountered as regards diffusion as far as the transfer of the gases liberated during the charging and recharging procedures, from one electrode to the other is concerned. The point is that the mechanism of such a transfer consists in dissolving said gases in liquid electrolyte contained in the pores of the electrode, wherein it is generated, in its diffusing in a dissolved state over the flooded pores of said electrode, of the separator and of thee opposite electrode, the reaction of its ionization occurring not until said operations are completed. It is due to very low solubility of hydrogen and oxygen in liquid electrolytes under standard conditions and thereby very low corresponding value of the diffusion coefficient that the resultant ionization rate of said gases on the opposite electrodes with a virtually completely filled void space of the separator and of both electrodes is very low, too. Said rate is likewise as low even in cases where one or both electrodes feature certain gas porosity whereas the separator pores are footed completely. Very low rate of transfer of the gases between the electrodes is much less their generation rate during recharging, whereby the pressure inside the capacitor casing increases which is fraught with its depressurization and even with blasting.
An inventive concept underlying the present invention consists in that a single system of gas pores is established in a DEL capacitor throughout the entire electrochemical group (ECGp) thereof, comprising porous electrodes and a porous separator. Thus, oxygen and hydrogen gases which are liberated during capacitor charging and recharging are conveyed very rapidly along said system to the opposite electrodes whereon both gases undergo ionization to form water or respective ions (H+, OH, and others). The fact is that the diffusion coefficients of gases in the gaseous phase is four orders of magnitude higher than that in the liquid phase. Such a system of gas pores is provided due to the fact that the void space of both porous electrodes and the porous separator having a degree of filling of their pores within 90 and 40%. Hence the proportion of a non-filled void space of gas pores (gas porosity) in each of the porous body of the ECGp falls within 10 and 60%, with the result that the required system of gas pores is established. Further reduction of the degree of filling the ECGp with electrolyte is undesirable as fraught with an appreciable increase in the internal resistance of the capacitor.
To establish gas porosity can be provided by a variety of techniques, one of which being applicable whenever electrolyte is contained only in the pores of the electrodes and of the separator, i.e., when no free electrolyte is present in the capacitor. The definite values of a degree of filling the void space in the electrodes and separator in the aforementioned range from 90 to 40% of a total space is attainable, firstly, due to appropriately measuring out a full amount of electrolyte introduced into the capacitor, and secondly, by using electrodes and a separator with definite mutually coordinated porous structures. As a matter of fact, distribution of a liquid inside a system of mutually contacting porous bodies depends quantitatively on the size distribution curves (porograms) of the pores of said porous bodies. The nature of said quantitative dependence has been established in the following papers (cf. Volfkovich Yu. M. the Journal xe2x80x9cElektrokhimiaxe2x80x9d, 1978, v. 14, #4, p.546, vol. 14, #6, p.831; #10, p. 1477 (in Russian); Volfkovich Yu. M. and Bagotzky V. S. Power Sources, 1994, v. 48, pp. 327, 339). For instance, with an increased proportion of large pores in a separator compared with electrodes, the degree of filling the separator pores is decreased compared with said electrodes. Control over the execution of the preset values of the degree of filling of pores in each porous body of the ECGp may be carried out, firstly, by weighing the separator and electrodes both in a fully flooded stated (under vacuum) and following a working impregnation of the separator and electrodes, assembling the capacitor and its subsequent disassembling; and secondly, by taking the porograms of the electrodes and separator, as well as by weighing the entire ECGp before and after impregnation with electrolyte.
In order to fulfil the aforestated condition as to electrolyte containing only in the pores of the electrodes and of the separator, it is reasonable that one capacitor or a bank of capacitor elements be held between the load-bearing cover of the casing as otherwise the capacitor internal resistance is increased.
Another method for providing the required gas porosity of the electrodes and separator consists in that a dispersed water repellant is added to one or both electrodes and/or to the separator appearing as, e.g., polytetrafluoroethylene or polyethylene. Water-repellency treatment of the negative electrode increases the rate of diffusion of the electrolyte-dissolved oxygen inside the pores directly to the internal electrode/electrolyte interface and the resultant higher rate of its electric reduction. Insofar as capacitor recharging as a result of misoperation (with E less than 0 V) must not be ruled out completely, hydrogen is liable to evolve on the negative electrode. Adding a dispersed water repellant to the positive electrode accelerates abruptly the process of hydrogen transporting to the inner surface thereof and the resultant process of hydrogen electric oxidation on said electrode. Thus, adding water repellants to the composition of porous electrodes helps solving the problem of creating completely pressurized capacitor.